Some people proposed that a laser diode be used as a light source for an image projector (which is often simply called a “projector”) that is designed to project an image onto a screen. A laser diode can emit light with higher color purity than an incandescent lamp for use in a conventional image projector. That is why it is expected that the image projected would have increased color reproducibility with the use of a laser diode.
Also, if a semiconductor laser diode is used, then a dichroic mirror, which is needed as a spectral element for a conventional incandescent lamp, and other optical elements can be omitted, thus simplifying and downsizing the structure of the optical system and increasing the optical efficiency. For that reason, it is expected that a smaller power-saving image projector would be realized with a semiconductor laser diode.
On top of that, if a laser diode is used as a light source, a sharply focused laser beam may be used as a scanning beam. That is why by using a laser diode as a light source for an image projector, a scanning image projector for producing an image by making a two-dimensional scan with a laser beam by way of a scanning means such as a mirror would also be realized.
The scanning image projector produces an image by changing the intensities of a laser beam. For that reason, the scanning image projector that uses a laser light source can save even more power than a normal image projector that uses a two-dimensional image display device such as an LCD panel or a DMD (digital mirror device). What is more, the scanning image projector needs no illumination optical system to illuminate a two-dimensional image device uniformly, and is expected to have an even smaller size.
By taking advantage of these beneficial features, the scanning laser image projector could also be built in a small mobile electronic unit. For example, even with a small mobile electronic unit such as a cellphone, which is usually hard to be equipped with a big monitor, the user can also enjoy a big screen display.
For example, Patent Document No. 1 discloses a laser scanning image projector for making a raster scan with a laser beam. Specifically, the image projector makes a horizontal scan back and forth and a vertical scan using a stepwise drive signal, and stops the vertical scan while making the horizontal scan.
On the other hand, Patent Document No. 2 discloses a laser scanning image projector that draws a Lissajous pattern with deflection frequencies f1 and f2 that satisfy f1:f2=n:m and that presents an image at a frequency fr (=f1/n=f2/m) at which the Lissajous pattern makes one round.
It is known that in presenting an image by projecting coherent light such as a laser beam onto a screen, the laser beams reflected from the projection plane would interfere with each other, thus producing speckle noise that would make the projected image look glaring or too dark or too bright locally.
As shown in FIG. 4, a projection plane 301 such as a screen is normally not completely flat but has some unevenness. If laser beams 302 and 303 are projected from a laser light source 304 onto such a projection plane 301, the reflected beams will intersect with each other at a point A because the projection plane 301 has unevenness. In this case, if the difference between the optical path lengths of the laser beams 302 and 303 as measured from the laser light source 304 to the point A is an integral multiple of the wavelength of the laser beam, then the laser beams 302 and 303 will enhance each other due to interference. On the other hand, if the difference between the optical path lengths of the laser beams 302 and 303 to the point A is an integral multiple of a half wavelength, then the laser beams 2 and 3 will weaken each other due to interference.
In this manner, the image comes to have some portions with excessively high luminances and some portions with excessively low luminances, thus producing speckle noise and debasing the quality of the image projected.
To reduce such speckle noise, Patent Document No. 3 proposes vibrating the screen with a gas flow and moving the unevenness at the surface of the screen.                Patent Document No. 1: Japanese Patent No. 2724016        Patent Document No. 2: U.S. Pat. No. 6,843,568        Patent Document No. 3: Japanese Patent Application Laid-Open Publication No. 2005-107150        